Interesting Woodworking Link

Tuesday, January 27, 2009

I am interested in hide glue for a number of reasons. The first of which is the "romance" of using a traditional method that has its foundations as far back as Ancient Egypt and the cabinet shops of Colonial America. I also appreciate the pace of using hide glue . . . slow while getting set-up and quick when applying the glue. I also like the reversibility of hide glue and the fact that it accepts stain and finish without leaving telltale glue marks like that pesky spot of PVA that you missed during final clean up. All that said, hide glue is not the main glue in my shop, but I am adding it to my shop armamentarium. You can experiment with this fascinating and historic glue inexpensively. Trust me, you won't regret the adventure!

A Hide Glue Primer

Properties of hide glue

Hide glue is thermoplastic - softens to a liquid state when warmed and solidifies when cool

Hide glue is hydroscopic - in other words, it has an affinity for water which means, for wood working purposes, it responds to some degree with changes in relative humidity just as wood does and more importantly, a hide glue joint can be "reversed" with warm water

Hide glue is an animal protein (collagen) - when hide glue cools (dries) the protein structure contracts (on a molecular level) which has the effect of clamping

Hide glue grading - hide glue is graded in terms of "gram strength" (GEEK FACTOID: gram strength is a measure of how many grams of force it requires to depress a 1/2” plunger 4mm. into a 12.5% protein solution of the glue at 10° C). Hide glue gram strengths range from 85 to 379. The important point to know is the higher the gram strength, the quicker the set. The general recommendation range for for woodworking applications is 135 - 251 gram strength (192 or 195 is considered the all around glue for most woodworking applications).

Some Work Flow Considerations for Using Hide Glue

Mix a large batch of hide glue and pour into a set of ice cube trays. Store the premixed "hide glue cubes" in the freezer. Add the cubes to the glue pot as needed to keep a batch of working hide glue ready at the bench.

The general rule of thumb for mixing hide glue is one cup of dry glue to one cup water, vary the amount of water depending on the desired consistency. Add the glue to the water and allow to soak for 30 minutes. Stir occasionally while gently heating to 140 F°. Be careful to not "cook" the glue as this will weaken the glue by breaking down the protein molecular chains.

On a day when you are planning on working with hide glue, the second thing to be turned on after the shop lights is the glue pot!

Your glue pot can be covered with plastic wrap and stored in the refrigerator overnight. After a week of use (heat-cool-heat) it is best to discard and start with a fresh batch.

Modifications to Hide Glue

To make hide glue liquid at room temperature (longer open time / slower drying) add approximately 5 tablespoons of urea for each cup of dry glue.

To make hide glue be made waterproof add 1/2-1% aluminum sulfate.

To make hide glue flexible add approximately 5% glycerin (based on the weight of glue), with increasing increments of 2-3% to achieve greater flexibility.

To increase the " wetting properties" of hide glue add approximately 1% vinegar.

The Basics of Hammer Veneering with Hot Hide Glue

Both surfaces are brushed with glue and placed together. The top veneer surface is also coated with glue to act as a lubricant. The "hammer" is then pushed across the veneer surface from the center outward to force out the excess glue. The glue will begin to cool and gel immediately and holding the veneer in place. For large work surfaces, make sure the surface is warm. Have a heat source (heat lamp) nearby for a longer working time.Hide Glue & Veneering References

The definitive hide glue resource and "hide glue guru" is Eugene Thorndahl of Bjorn Industries in North Carolina. Eugene has the most experience with hide glue. Bjorn Industries has the widest variety available of hide glue gram strengths.

4 comments:

As you have stated this is very widely used in the world of lutherie. In those circles whether you use hide glue or PVA can be as much of a religious argument as pins vs. tails in your dovetails!

I really considered trying it when I started the guitar project (it's in the state you last saw it, waiting to be finished - in case you were wondering).

It is very useful in instrument making because of both it's strength and also it's reversibility. Guitars can come under a lot of stress with the string tension and the potential drastic weather changes (i.e. in a case in a hot car and then into an air conditioned auditorium - you get the picture).

Typically for lutherie they use 192 gram strength glue. However, they really caution using urea to get longer open time and to keep it liquid at room temperature (this is exactly how Franklin does it with their liquid hide glue product). Evidently the urea weakens the glue bond at least as far as whats required for stringed instruments.

I know absolutly nothing about hide glue except for all the negative things my upholster brother has to say about the furniture that comes into his shop that has basically fallen apart because the hide glue failed.

That's not a knock against hide glue, just passing along his criticism as someone who deals with the remnants of bad side.

I for one enjoyed your primer post because it was a good introduction for me. It looks like something I think I may try.

Thank you for posting this and you other informative posts - you're doing a great service here to your fellow woodworkers!

Thanks for the comment and insight from your brother. I must say my experience is on the learning curve of things so it will be good to have this in mind. I do know from talking with a few guys that have experience with period furniture that there is some very old furniture with very solid hide glue joints.

By the way, I am a huge fan of Eagle Lake Woodworking! You have shared some great stuff recently, especially your Super Sled and accessrories.

One of the reasons I've seen/read about a number of times for using Hide Glue is that furniture can later (if necesary) be taken apart to carry out repairs.

I'm presently trying to remove some turned wood knobs that are hide glued on to a 1950ish Oak chest so I can re-finish the badly deteriorated varnish. Have tested some of excess glue around corners of drawers, and softens OK like Hide Glue. But getting the knobs off doesn't seem so simple.

I'd appreciate any ideas on how to melt the glue hidden in behind the knobs without damaging the drawer fronts?